Nanotubes Trigger Neurons.

By Prachi Patel-Predd.
September 2, 2006.

Using carbon nanotubes as small electrodes might one day lead to safe and
effective retinal implants.

Researchers at Stanford University have used electrodes made of bundles of
multiwalled carbon nanotubes to stimulate rat neurons. In a Nano Letters
paper published online this week, the researchers describe making arrays of
the 50-micrometer electrodes on a silicon substrate and growing the neurons
on the arrays. The neurons responded consistently to the electrical signals
from the electrodes.

The experiment is an advance toward the long-term goal of using neural
prosthetics, such as cochlear and retinal implants, to address individual
neurons. Neural prosthetics that restore vision or hearing typically use
implanted microelectrode arrays to send electrical signals to nerve cells or
directly to the brain. While cochlear implants are already in use,
scientists are in the process of developing artificial retinas. A retinal
prosthetic, for instance, would have an array implanted near the retina,
stimulating the nerve cells that send signals to the optic nerve.

The problem so far with making safe, effective implants has been the
electrodes, which are made of metals such as platinum and iridium. To
effectively transfer electrical current to the neurons, electrodes made of
platinum, for example, have to be larger--by hundreds of micrometers--than
the cells they are trying to stimulate. This makes it hard to address
specific cells. Other metals can be used for smaller electrodes, but they
can sometimes dissolve or cause chemical reactions that harm the surrounding
tissue.

Besides, the human body sometimes treats long-term implants that use metals
as foreign objects. What's more, the relatively large metal electrodes are
rigid and can damage the soft tissue in which they are implanted. "Often,
what happens in standard electrical prosthetics is you get an inflammatory
reaction that can basically form a scar tissue around the implant," says
Harvey Fishman, an ophthalmologist and neuroscientist who led the work at
Stanford University, along with Ke Wang, a graduate student in applied
physics.

Fishman, who is now at the Plager Vision Center in Santa Cruz, CA, says that
the human body would not treat implants containing carbon nanotube
electrodes as foreign objects. "It's probably one of the safest materials to
use, because carbon naturally occurs in the body," he says. "We're mostly
made of carbon and water." In addition, the Stanford team could make very
small electrodes, 50 or 100 micrometers in diameter, out of the carbon
nanotubes, which are tough, flexible, and good electrical conductors.

Other researchers are also seeking ways to include carbon in the electrical
interface of neural prosthetics. Todd Pappas at the University of Texas
Medical Branch and his colleagues at Rice University have successfully
stimulated rat neurons grown on carbon nanotube mats made of horizontally
grown nanotubes. The advantage of having an electrode that juts out
vertically from a surface is that you can place the charge where you want
it, Pappas says. But he thinks that the small electrodes could be a problem
when implanted in a living organ--they will not be easy to couple to
individual nerve cells. For that, he says the researchers will have to take
additional steps--guiding the neurons to grow towards the electrodes, for
example, or covering the surface with a more cell-friendly material.

Meanwhile, other researchers are adapting the microelectrode technology
specifically for retinal implants. Orlando Auciello and his colleagues at
Argonne National Lab are working on making silicon microchips bio-inert by
incorporating carbon, albeit in another form--a material they have developed
called ultrananocrystalline diamond. While silicon cannot be placed directly
in the eye, a chip encapsulated with this material could be, Auciello says.
The device is now in clinical trials. Since the diamond-coating technology
has not been fully developed yet, each of the six patients in the clinical
trial had a microchip implanted on the side of the head that is connected to
a platinum electrode array in the retina. The electrodes are 500 micrometers
in diameter.

With the smaller carbon nanotube electrodes, Auciello says, "You can address
much lower number of cells with one electrode, even if it doesn't go to a
single cell." The Stanford work opens up the possibility of integrating
carbon nanotubes on the silicon microchip, he says. "This new development is
very exciting if it can be reproduced."